Carboxypeptidase G1 is immobilized in hollow fiber capsules, hollow fiber cartridges and tubular bundle reactors. The folate depleting and the concomitant antitumor activity of such devices in experimental animals is investigated. The rapid removal of methotrexate from blood is studied. The characterization of the enzyme from various sources and the study of the kinetic behavior of both the soluble and the immobilized enzymes are also carried out.